Recently, the number of people using the Internet has rapidly increased, and thus the importance of security and the importance of reducing the traffic load are increasing. For example, even when the Internet is used as a leased line by using VPN (Virtual Private Network) technology, to ensure security, encryption of data to be transmitted and received is recommended to prepare for the possibility of crosstalk. However, a large amount of arithmetic processing is required for encryption and decryption with sufficient strength, and thus, there is a problem in that videos and images cannot be encrypted with sufficient strength. Furthermore, in conventional techniques, in order to reduce the traffic load on the Internet, when a video or an image is transmitted, the amount of data is often compressed to several percent to several tens of percent by using an irreversible compression technology, such as MPEG4, thus causing a problem in that the quality of the video or the image obtained after transmission and expanding is reduced.
One of methods of encrypting and decrypting images is a technique invented and patent-registered (PTL 1) in Japan by the inventor of the present invention. In this method, a digital image is transferred by using a known transfer function, to remove a particular frequency in frequency space and to be encrypted, and iterative operations are performed on the basis of a Bayse probabilistic formula from the distribution of the encrypted data and the transfer function used for encryption, thus restoring the digital image. However, this method merely reduces the amount of image data in frequency space and has a problem in that there is no change in the data amount itself to be transmitted, and thus, the traffic load remains unchanged.
Thus, the inventor of the present invention has invented and filed a patent for a technique of transmitting images and videos over a network while reducing the traffic load (PTL 2). In this method, image and video data is smoothed to remove information corresponding to a high-frequency portion, is coded by using a compression technology, such as JPEG or MPEG, is transmitted, and is decoded, at the receiving side, by using an expanding technology, such as JPEG or MPEG, corresponding to the method used at the transmitting side, and then image restoration is performed by using the Richardson-Lucy method or the like. However, although this method exhibits a high compression ratio up to 1/100, an image is divided into microblocks and is quantized, and thus there is concern about the occurrence of noise that would be caused by enhancement of block boundaries at the time of image restoration, in some cases. Furthermore, transmitted images are accumulated, and thus there is a problem in that this method lacks real-time characteristics and is not suitable for video processing.
PTL 3 discloses a method in which the encryption method of PTL 1 is used to reduce the resolution of an image, thus degrading the image, and to encrypt the image. In PTL 3, the resolution of an image is degraded to a level beyond that at which, for example; the number plate of a vehicle can be recognized, by using the encryption method of PTL 1, the image is transmitted to a receiving device over a LAN (Local Area Network) or the Internet, and, at the receiving device, iterative operations are performed from only information of the received degraded single image by using an encrypted-data decrypting device of PTL 1, thus restoring the image. However, as described in PTL 3, the technique of PTL 3 has a configuration in which the transfer function used for encryption is not delivered every time and thus has a constraint problem in that the transfer function used for encryption must be known, a problem in that restoration of the transfer function is required to restore the degraded image, a problem in that the transfer function used for encryption needs to be fixed to a known function, thus weakening the security, and concerns that a failure to identify the transfer function may result in a restoration failure.
Methods for compressing, encrypting, and transmitting an image include encryption facsimile. For example, in a method of PTL 4, a document is read as an image, the image is divided into blocks, is compressed, and is encrypted by using an encryption key, in units of blocks, and the blocks are arrayed, with read-control patterns being interposed therebetween, to configure an image document. Then, the image document is combined with a plain text document that can be public, such as a delivery address, and is then transmitted. At the receiving side, a tilt or displacement of the received document is corrected, the document is printed, the printed document is re-read by the receiving device, an encryption key is specified to decrypt and expand a graphic at an encrypted portion, and a reproduced document is printed. The method of PTL 4 has a problem in that an image is subjected to a change due to garbled bits during transmission, whereas the encrypted portion has a complicated patterned image, which is resistant to leakage during transmission; a problem in that this method cannot be used to transmit color images; a problem in that data is not packetized, and thus this method is not suitable for Internet communications; a problem in that this method is not always a continuous transmission means; and a problem in that printing is required once, which complicates the operation.